Laser welding of thick stainless steel with 20 kW CO2 laser was studied. The focal point shifts at high output power. For describing welding position the distance h between the center of focusing mirror and surface of test piece was used as the welding position reference, and the seam formation and welding depth were also studied. Besides, high power laser welding was more sensitive to the focal system. The seam formation gave little differences at different welding positions when the focal length f was 300 mm while it was not the case when the f=200 mm. A good bead was got with one pass welding when f=200 mm, h=208 mm, laser power was 18 kW, and welding velocity was 2 m/min. The results illustrated that through optimizing welding parameters, without groove and filling materials, 12 mm thick stainless steel plate was butt welded with 20 kW laser and the seam formation was well.